Antenna system with distortion compensating reflectors

ABSTRACT

D R A W I N G AN ANTENNA SYSTEM HAS A REFLECTOR SYSTEM COMPRISING A PLURALITY OF REFLECTORS HAVING ROTATIONALLY ASYMMETRICAL CURVED SURFACES SUCH AS ELLIPTIC, HYPERBOLIC OR PARABOLIC SURFACE GEOMETRIES. IN ORDER TO COMPENSATE FOR THE ASYMMETRICAL PROPERTIES OF THE REFLECTORS, AT LEAST ONE PAIR OF ASYMMETRICAL REFLECTORS IS ARRANGED IN FACING RELATIONSHIP SO THAT THE DIFFERENCES IN ASYMMETRICAL PROPERTIES ARE COMPENSATED BY THE OPPOSITE RADIATION DISTRIBUTION CHARACTERISTICS OF EACH REFLECTOR. THE REFLECTORS ARE POSITIONED APART SO THAT AMONG THE REFLECTORS THE RATIO BETWEEN THE PRODUCT OF THE DISTANCES FROM EACH REFLECTION POINT OF EACH REFLECTOR TO THE FOCUS OF EACH REFLECTED BEAM FORMS A CONSTANT RATIO WITH THE PRODUCT OF THE DISTANCE FROM THE REFLECTION POINT TO THE FOCUS OF EACH INCIDENT BEAM. ACCORDINGLY, WHEN AN ELECTROMAGNETIC BEAM SOURCE WITH A ROTATIONALLY SYMMETRICAL BEAM, IS APPLIED, A ROTATIONALLY SYMMETRICAL RADIATION DISTRIBUTION CHARACTERISTIC RESULTS AT AN APERTURE OF THE ANTENNA SYSTEM EVEN THOUGH THE REFLECTORS HAVE ROTATIONALLY ASYMMETRICAL CURVED SURFACES.

United States Patent 9 Mizusawa et al.

[11] 3,821,746 June 28, 1974 ANTENNA SYSTEM WITH DISTORTION COMPENSATING REFLECTORS [73] Assignee: Mitsubishi Denki Kabushiki Kaisha,

Tokyo. Japan Filed: Nov. 2, i972 21 Appl.No.:303,049

[30] Foreign Application Priority Data OTHER PUBLICATIONS Kitsurgawa; Design of the Beam-Waveguide Primary Radiators of the Cassegrain Antennas for Sat. Comm.; I970 GAP International Symposium, Columbus Ohio; Sept. 14, 1970.

Primary Examiner- Eli Lieberman Attorney, Agent, or Firm-Oblon, Fisher, Spivak, Mc- Clelland & Maier 5 7 ABSTRACT An antenna system has a reflector system comprising a plurality of reflectors having rotationally asymmetrical curved surfaces such as elliptic, hyperbolic or parabolic surface geometries. In order to compensate for the asymmetrical properties of the reflectors, at least one pair of asymmetrical reflectors is arranged in facing relationship so that the differences in asymmetrical properties are compensated by the opposite radiation distribution characteristics of each reflector. The reflectors are positioned apart so that among the reflectors the ratio between the product of the distances from each reflection point of each reflector t0 the focus of each reflected beam forms a constant ratio with the product of the distances from the reflection point to the focus of each incident beam. Accordingly, when an electromagnetic beam source with a rotationally symmetrical beam, is applied, a rotationally symmetrical radiation distribution characteristic results at an aperture of the antenna system even though the reflectors have rotationally asymmetrical curved surfaces.

5 Claims, 7 Drawing Figures la F2 A PRICR ART FIG.3

PATENTEDmza 1914' SHEET 2 BF 2 FIG.7

ANTENNA SYSTEM WITH DISTORTION COMPENSATING REFLECTORS BACKGROUND OF THE INVENTION 1. Field Of The Invention:

This invention relates to an improvement of an antenna system having rotationally asymmetrical reflectors. By a rotationally asymmetrical reflector is meant a reflector which, due to its surface shape, causes an incident electromagetic field to be reflected asymmetrically so that an incident symmetrical electromagnetic field distribution loses its symmetry when it is reflected. For example, any of a parabolic reflector, a spherical reflector a hyprobolic reflector or an elliptic reflector are considered to be rotationally asymmetrical. On the other hand, a flat reflector is considered to be rotationally symmetrical.

2. Description Of The Prior Art:

I-Ieretofore, a typical conventional antenna having rotationally asymmetrical reflectors has been the offset Cassegrain antenna shown in FIG. 1.

The Cassegrain antenna comprises a rotationally asymmetrical hyperboloid reflector la having a foci F F a rotationally asymmetrical paraboloid reflector lb having a focus F and a horn 2. Since a rotationally asymmetrical reflector is used, the electric field distribution on the aperture surface A-A' of the antenna is i a rotationally asymmetrical distribution when the beam reaches the antenna aperture surface A-A through the hyperboloid reflector la and the paraboloid reflector 1b, even though a rotationally symmetrical beam is fed from the horn 2.

The asymmetrical distribution at the'aperture A-A causes a deterioration of some antenna characteristics,

e.g., decrease of gain, increase of side lobe radiation,

and increase of undesired polarized wave radiation.

I-Ieretofore, in order to decrease the asymmetrical distribution, the focal lengths of the hyperboloid reflector la and the paraboloid reflector 1b have been made as long as possible. Although this technique has been somewhat successful, it is difficult to remove the asymmetrical distribution because, while a very long distance between the hyperboloid reflector 1a and the paraboloid reflector is required, the distance is limited by structural restrictions.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a new and improved unique antenna system combining successively a plurality of rotationally asymmetrical reflectors whereby the difference in asymmetrical properties is compensated by opposite radiation distribution characteristics so that the electric field distribu tion at the antenna aperture is rotationally symmetrical.

It is another object of this invention to provide a new and improved unique antenna system having two rotationally asymmetrical reflectors, which compensate for each others asymmetrical properties; and a primary feed system, wherein the beam radiated to the antenna becomes rotationally symmetrical independent of the elevation rotation of the antenna by using a flat reflector moving with the elevation rotation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a conventional antenna system having rotationally asymmetrical reflectors;

FIG. 2 is a schematic view of an antenna to illustrate the function of the antenna of this invention;

FIG. 3 is a schematic view of one preferred embodiment of the antenna in accordance with this invention;

FIG. 4 is a schematic view of another preferred embodiment of the antenna in accordance with this invention;

FIG. 5 is a schematic view of an embodiment using the system of this invention for an antenna primary feed system;

FIG. 6 is a schematic view of a preferred embodiment using the system of this invention for a primary radiator of an elevation-azimuth mount type antenna.

FIG. 7 is a schematic view of a modification of the embodiment of the antenna system of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS and then diverged,as shown by the arrow lines of FIG.

The relation of power density between the incident beam to the reflector and the reflected beam is expressed by the Equation 1 as hereinafter shown.

The power contained in each of the solid angles d0, and dQ are P dfl andP dfl respectively. Since the powers are equal, the relationship can be expressed as wherein P designates the power density of an incident beam fed to the reflector per solid angle unit;

P designates the power density of a reflected beam per solid angle unit; d0, designates the differential solid angle bounded by the vectors F, M and F, M at the F 1 side; and

dfl designates the differential solid angle bounded by the vectors F M and F M at the F side. When the normal vector which is perpendicular to thesurface of the reflector 3 and exists at every point on the surface 3 to define the shape thereof is shown by m as in FIG. 2, the angle 0 between the normal vector and the incident beam is the same as the angle ObetWeen the normal vector and the re-' flected beam. Since the length of an arc ween the points M and M on the reflector 3 is F 1 M d0, divided by cos 0, where m is a vector representing the distance and direction between the points F and M, the area on the reflector encompassed by the solid angle d0, can be expressed as F 1 M (190/0050 As can be seen in FIG. 2, each area The relation between the power density of the reflected beam and that of the incident beam per solid angle unit can be derived from the Equations 1 and 2 (3) When a uniform incident beam is fed to the reflector 3, the power density of the reflected beam changes under the Equation 3 according to the direction of the incident beam. This results in an asymmetrical electric field distribution being produced at an aperture of the antenna even if the-radiating source is symmetrical because the surface of the reflector 3 is rotationally asymmetrical, that is, is not flat.

. When a paraboloid reflector such as is shown in FIG.

' 1 is employed, F of FIG. 2 can be considered to be of an infinite distance.

Also, the value F M can be considered constant for any beam. Accordingly, since the numerator in Equation 3 is constant the Equation 3 can be modified to PZ/PI I Hm The power density of the reflected beam changes depending upon the length F l M of the incident beam, and an asymmetrical beam results even if the incident beam is symmetrical. I

In order to provide a constant power density regardless of the direction of the beam so that the symmetry of the incident beam is not lost, P /P in Equation 3 should be constant for any beam. In order to provide a constant P /P in the Equation3 when one reflector where P, designates the power density of an arbitrary incident beam fed to a reflector 4a, and P designates the power density of a corresponding beam reflected from the reflector 4b. i

In order to provide a uniform beam at the point F when a uniform beam is supplied to the reflector 4a, F ll, in Equation 5 should be constant.

In the embodiment, the reflectors are arranged to be symmetrical-about the point F accordingly,

l l a z and g I FZMI =F2M2 4 Consequently the Equation 5 is modified to A beam having the same shape as the beam from F 1 to M is radiated from the point F so that the symmetry of a symmetrical electric field distribution is not lost due to the rotationally asymmetrical reflectors 4a and 4b.

In the above embodiment, only two reflectors are employed. It should be understood that similar considerations can be applied to a system having many reflectors as shown in FIG. 4.-The ratio of the power density P of the beam reflected from the final reflector to the power density P, of the incident beam fed to the first reflector can be derived from the distance between each reflection point of each reflector and each focus.

P /P 1R,1R Jan/ 11,11 11,,

The ration P lP is constant when the ratio of the prodnets of the lengths in the numerator and denominator of the Equation 9 is designed to be constant for any beam, and rotational symmetry is not lost.

The antenna system of this invention can be used as a. primary feed system for other types of antennas or electromagnetic energy radiators e.g., a lens 5 as shown in FIG. 5. It should be understood that the antenna system of this-invention is also effective as a primary feed system for an antenna having fixed input and output ends and further having elevation and azimuth rotations. Refering now to FIG. 6, an embodiment is shown wherein the antenna system consists of a primary feed system having a horn 4a a flat reflector 4b, a paraboloid reflector 4c, a paraboloid reflector 4d, a flat reflector 4e, a sub-reflector 2 and a main reflector 3. The two paraboloid reflectors 4c'and 4d have rotationally asymmetrical curved surfaces and the same surface-shape and are arranged so as to symmetrically face each other. The sub-reflector 2 and the main-reflector 3 have respectively rotationally'symmetrical surfaces.

In the antennasystem, the flat reflector 4e, the subreflector 2 and the main-reflector 3 are turned around the elevation axis B-B'. The horn 4a is fixed and all other reflectors are turned around the azimuth axis A-A. I

The beam transmitted through the primary feed system is transmitted by focusing to the center line between the reflectors. Accordingly, no rotary joint is required as in aconventional antenna. The beam fed from the input P is transmitted through the horn4a, the flat reflector 4b, the paraboloid reflector 4c, the paraboloid reflector 4d, the flat reflector 4e, the sub-reflector 2 the main reflector 3, and is finally radiated into space, as shown by the broken line in FIG. 6.

The electric field distribution of the beam from the horn 4a is rotationally symmetrical, and that of the beam fed from the flat reflector 4b to the paraboloid reflector 4c is also rotationally symmetrical.

The electric field distribution of the beam reflected from the paraboloid reflector 4c is rotationally asymmetrical. When it is fed to the next paraboloid reflector 4d, the rotationally asymmetrical distribution is compensated by the asymmetrical structure of the paraboloid reflector 4d. Thus the electric field distribution of the beam reflected from the paraboloid reflector 4d becomes rotationally symmetrical.

The sub-reflector 2 and the main-reflector 3 are respectively rotationally symmetrical, so that the electric field distribution at the antenna aperture surface 8-8 is also rotationally symmetrical. When the elevation angle of the antenna is changed, the rotational symmetry of the field distribution of the beam reflected from the flat reflector 4e can be maintained because the reflector 4e turning with the elevation angle is a flat reflector in the embodiment.

The embodiment of FIG. 6 is effective when geometrical optics are applied. Instead of using the paraboloid reflectors 4c and 4d as shown in FIG. 6, it is possible to dispose a pair of ellipsoid reflectors 4c and 4d whose surfaces have the same curvature and face each other symmetrically as shown in FIG. 7. This replacementis effective for a system which is used in a relatively low frequency band and in which a consideration based on electromagnetic theory is required. That is, in the case wherein the incident wave to the reflector system and the reflected wave from the reflector are apt to largely diverge as shown by the broken line of FIG. 7, the system of FIG. 7 is effective. The phase e enters of the incident wave on the ellipsoid reflector 4c and the reflected wave from the ellipsoid reflector 40', that is, the center F of the wave front of the incident wave and the center F of that of the reflected wave, are designated as the twofoci of the ellipsoid reflector 4c.

The relation between the power P, of the incident wave fed to the reflector system of FIG. 6, per solid angle unit, and the power P of the reflected wave, per solid angle unit, can be derived from the Equation 5 when the points F and F are distant from each reflector. I

When the two ellipsoid reflectors have the same shape and are arranged to be bilaterally symmetrical, the relation between the powers P and P can be expressed by the following equation.

Therefore, the relation between the incident wave and the reflected wave for any beam is not changed. When the incident wave having a rotationally symmetrical field distribution is fed into the reflector system, the reflected wave has a rotationally symmetrical field distribution.

In the above-mentioned embodiment, the two rotationally asymmetrical reflectors face each other. However, in case many rotationally asymmetrical reflectors, that is, more than two, are employed and, in addition, a pair of the rotationally asymmetrical reflectors face each other through a pair of flat reflectors, the abovementioned relations and features can be maintained.

As stated above, in accordance with this invention,

the relation between the power densities of the incident wave and of the reflected wave can be kept constant and a rotationally symmetrical field distribution can be provided, even though rotationally asymmetrical reantenna and decrease of side lobe and undesired polarized wave can be achieved.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the'invention may be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. An antenna system comprising a reflector system including:

a plurality of reflectors having rotationally asymmetrical curved surfaces; and

a horn used as the primary radiator to generate a symmetrical electric field;

at least one pair of asymmetrical reflectors arranged in facing relationship whereby the differences in asymmetrical properties are compensated by opposite radiation distribution characteristics so that the electric field distribution at an antenna aperture is rotationally symmetrical;

said plurality of reflectors positioned to provide a constant ratio between the product of the distances from each reflected point of each reflected beam on each reflector to each focus of the reflected beam and the product of the distances from said reflected points to each focus of each incident beam with regard to any beam passed from the horn and said reflectors to the antenna aperture.

2. An antenna system comprising:

a primary feed system including:

a plurality of reflectors; and

a horn to generate a symmetrical electric field;

at least one pair of asymmetrical reflectors arranged in facing relationship whereby the differences in asymmetrical properties are compensated by opposite radiation distribution characteristics so that the electric field distribution at an antenna aperture is rotationally symmetrical; said horn having input and output ends fixed for elevation and azimuth rotation of the antenna wherein reflectors turning with an elevation rotation are flat reflectors while the reflectors independent of the elevation rotation are a plurality of rotationally asymmetrical curved surface reflectors said plurality of curved reflectors positioned to provide a constant ratio between the product of the distances from each reflected point of each reflected beam on each reflector to each focus of the reflected beam and the product of the distances from said reflected points to each focus of each incident beam in the primary feed system.

. 3. The antenna system according to claim 2, wherein said rotationally asymmetrical curved reflectors are paraboloid reflectors;

4. The antenna system according to claim 2, wherein said rotationally asymmetrical curved reflectors are ellipsoid reflectors.

5. The antenna system according to claim 2, wherein said antenna system further comprises a primary feed system including a horn, a first flat reflector, a paraboloid reflector, a paraboloid reflector, a second flat reflector, a sub-reflector and a main-reflector, the flat relfectors, the sub-reflector, and the main-reflector being tumable around an elevation axis, the said reflectors being tumable around an azimuth rotary'axis. 

